One class of digitizers utilizes an inductive winding in the table and an inductive winding in the cursor with inductive coupling therebetween. An excitation source is connected with one of the windings and a signal processing circuit is connected with the other to obtain position information. The winding in the table comprises a two-phase grid winding for each coordinate axis with each phase of a grid winding being comprised of parallel, equally spaced conductors. One of the windings is driven with a sinusoidal excitation current and a signal from the processing circuit is of constant amplitude and has a variable phase angle which varies with the position of the cursor relative to the grid winding. Means are provided to measure the phase angle as an indication of cursor position. This system is used for obtaining small scale or fine position measurement and additional means are provided for large scale or coarse position measurement. A digitizer of this type is disclosed in the Bailey U.S. Pat. No. 3,647,963. Similar digitizers are disclosed in Bailey U.S. Pat. No. 3,801,733, Freedman U.S. Pat. No. 3,699,253 and the Centner et al. U.S. Pat. No. 3,735,044. The prior art devices of this type require the use of analog circuits for system excitation. Additionally, the coarse measurement means is of the incremental type, such as that based upon counting cycles; this requires each measurement to be made with the cursor starting from a given reference position.
Another class of digitizers uses pulse excitation of either the cursor winding or the table winding. In one form, plural coils in the table produce individual output signals which, taken collectively, constitute a binary signal representing the position of the cursor. A digitizer of this type is disclosed in the Lewin U.S. Pat. No. 3,466,646 and a similar system is disclosed in the Inokuchi U.S. Pat. No. 3,819,857. This device requires a large number of conductors and the accuracy depends upon conductor placement. In another form, as shown in Kamm et al U.S. Pat. No. 3,904,822, multiple conductors in the table are sequentially pulsed and the cursor signal is processed to obtain position information. This is useful for only small distance and is of limited accuracy.
An additional class of digitizers utilizes capacitive coupling between a cursor and a conductive sheet or grid. Such a device is disclosed in the Asano et al U.S. Pat. No. 3,591,718. Similar devices are disclosed in the Dym et al U.S. Pat. No. 3,593,115, Dym U.S. Pat. No. 3,668,313 and the Ellis et al U.S. Pat. No. 3,567,859. This type of device affords only moderate accuracy over relatively small distances.